Network Working Group V. Fuller
Internet-Draft D. Farinacci
Intended status: Experimental D. Meyer
Expires: June 8, 2012 D. Lewis
Cisco
December 6, 2011
LISP Alternative Topology (LISP+ALT)draft-ietf-lisp-alt-10.txt
Abstract
This document describes a simple distributed index system to be used
by a Locator/ID Separation Protocol (LISP) Ingress Tunnel Router
(ITR) or Map Resolver (MR) to find the Egress Tunnel Router (ETR)
which holds the mapping information for a particular Endpoint
Identifier (EID). The MR can then query that ETR to obtain the
actual mapping information, which consists of a list of Routing
Locators (RLOCs) for the EID. Termed the Alternative Logical
Topology (ALT), the index is built as an overlay network on the
public Internet using the Border Gateway Protocol (BGP) and the
Generic Routing Encapsulation (GRE).
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 8, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20111. Introduction
This document describes the LISP+ALT system, used by a [LISP] ITR or
MR to find the ETR that holds the RLOC mapping information for a
particular EID. The ALT network is built using the Border Gateway
Protocol (BGP, [RFC4271]), the BGP multi-protocol extension
[RFC4760], and the Generic Routing Encapsulation (GRE, [RFC2784]) to
construct an overlay network of devices (ALT Routers) which operate
on EID-prefixes and use EIDs as forwarding destinations.
ALT Routers advertise hierarchically-delegated segments of the EID
namespace (i.e., prefixes) toward the rest of the ALT; they also
forward traffic destined for an EID covered by one of those prefixes
toward the network element that is authoritative for that EID and is
the origin of the BGP advertisement for that EID-prefix. An Ingress
Tunnel Router (ITR) uses this overlay to send a LISP Map-Request
(defined in [LISP]) to the Egress Tunnel Router (ETR) that holds the
EID-to-RLOC mapping for a matching EID-prefix. In most cases, an ITR
does not connect directly to the overlay network but instead sends
Map-Requests via a Map-Resolver (described in [LISP-MS]) which does.
Likewise, in most cases, an ETR does not connect directly to the
overlay network but instead registers its EID-prefixes with a Map-
Server that advertises those EID-prefixes on to the ALT and forwards
Map-Requests for them to the ETR.
It is important to note that the ALT does not distribute actual EID-
to-RLOC mappings. What it does provide is a forwarding path from an
ITR (or MR) which requires an EID-to-RLOC mapping to an ETR which
holds that mapping. The ITR/MR uses this path to send an ALT
Datagram (see Section 3) to an ETR which then responds with a Map-
Reply containing the needed mapping information.
One design goal for LISP+ALT is to use existing technology wherever
possible. To this end, the ALT is intended to be built using off-
the-shelf routers which already implement the required protocols (BGP
and GRE); little, if any, LISP-specific modifications should be
needed for such devices to be deployed on the ALT (see Section 7 for
aggregation requirements). Note, though, that organizational and
operational considerations suggest that ALT Routers be both logically
and physically separate from the "native" Internet packet transport
system; deploying this overlay on those routers which are already
participating in the global routing system and actively forwarding
Internet traffic is not recommended.
This specification is experimental, and there are areas where further
experience is needed to understand the best implementation strategy,
operational model, and effects on Internet operations. These areas
include:
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o application effects of on-demand route map discovery
o tradeoff in connection setup time vs. ALT design and performance
when using a Map Request instead of carring initial user data in a
Data Probe
o best practical ways to build ALT hierarchies
o effects of route leakage from ALT to the current Internet,
particularly for LISP-to-non-LISP interworking
o effects of exceptional situations, such as denial-of-service
attacks
Experimentation, measurements, and deployment experience on these
aspects is appreciated. While these issues are conceptually well-
understood (e.g. an ALT lookup causes potential delay for the first
packet destined to a given network), the real-world operational
effects are much less clear.
The remainder of this document is organized as follows: Section 2
provides the definitions of terms used in this document. Section 3
outlines the LISP ALT model, where EID prefixes are routed across an
overlay network. Section 4 provides a basic overview of the LISP
Alternate Topology architecture, and Section 5 describes how the ALT
uses BGP to propagate Endpoint Identifier reachability over the
overlay network and Section 6 describes other considerations for
using BGP on the ALT. Section 7 describes the construction of the
ALT aggregation hierarchy, and Section 8 discusses how LISP+ALT
elements are connected to form the overlay network.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20112. Definition of Terms
This section provides high-level definitions of LISP concepts and
components involved with and affected by LISP+ALT.
Alternative Logical Topology (ALT): The virtual overlay network
made up of tunnels between LISP+ALT Routers. The Border Gateway
Protocol (BGP) runs between ALT Routers and is used to carry
reachability information for EID-prefixes. The ALT provides a way
to forward Map-Requests (and, if supported, Data Probes) toward
the ETR that "owns" an EID-prefix. As a tunneled overlay, its
performance is expected to be quite limited so use of it to
forward high-bandwidth flows of Data Probes is strongly
discouraged (see Section 3.3 for additional discussion).
ALT Router: The devices which run on the ALT. The ALT is a static
network built using tunnels between ALT Routers. These routers
are deployed in a roughly-hierarchical mesh in which routers at
each level in the topology are responsible for aggregating EID-
prefixes learned from those logically "below" them and advertising
summary prefixes to those logically "above" them. Prefix learning
and propagation between ALT Routers is done using BGP. An ALT
Router at the lowest level, or "edge" of the ALT, learns EID-
prefixes from its "client" ETRs. See Section 3.1 for a
description of how EID-prefixes are learned at the "edge" of the
ALT. See also Section 6 for details on how BGP is configured
between the different network elements. When an ALT Router
receives an ALT Datagram, it looks up the destination EID in its
forwarding table (composed of EID prefix routes it learned from
neighboring ALT Routers) and forwards it to the logical next-hop
on the overlay network.
Endpoint ID (EID): A 32-bit (for IPv4) or 128-bit (for ipv6) value
used to identify the ultimate source or destination for a LISP-
encapsulated packet. See [LISP] for details.
EID-prefix: A set of EIDs delegated in a power-of-two block. EID-
prefixes are routed on the ALT (not on the global Internet) and
are expected to be assigned in a hierarchical manner such that
they can be aggregated by ALT Routers. Such a block is
characterized by a prefix and a length. Note that while the ALT
routing system considers an EID-prefix to be an opaque block of
EIDs, an end site may put site-local, topologically-relevant
structure (subnetting) into an EID-prefix for intra-site routing.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 2011
Aggregated EID-prefixes: A set of individual EID-prefixes that have
been aggregated in the [RFC4632] sense.
Map Server (MS): An edge ALT Router that provides a registration
function for non-ALT-connected ETRs, originates EID-prefixes into
the ALT on behalf of those ETRs, and forwards Map-Requests to
them. See [LISP-MS] for details.
Map Resolver (MR): An edge ALT Router that accepts an Encapsulated
Map-Request from a non-ALT-connected ITR, decapsulates it, and
forwards it on to the ALT toward the ETR which owns the requested
EID-prefix. See [LISP-MS] for details.
Ingress Tunnel Router (ITR): A router which sends LISP Map-
Requests or encapsulates IP datagrams with LISP headers, as
defined in [LISP]. In this document, the term refers to any
device implementing ITR functionality, including a Proxy-ITR (see
[LISP-IW]). Under some circumstances, a LISP Map Resolver may
also originate Map-Requests (see [LISP-MS]).
Egress Tunnel Router (ETR): A router which sends LISP Map-Replies
in response to LISP Map-Requests and decapsulates LISP-
encapsulated IP datagrams for delivery to end systems, as defined
in [LISP]. In this document, the term refers to any device
implementing ETR functionality, including a Proxy-ETR (see
[LISP-IW]). Under some circumstances, a LISP Map Server may also
respond to Map-Requests (see [LISP-MS]).
Routing Locator (RLOC): A routable IP address for a LISP tunnel
router (ITR or ETR). Interchangeably referred to as a "locator"
in this document. An RLOC is also the output of an EID-to-RLOC
mapping lookup; an EID-prefix maps to one or more RLOCs.
Typically, RLOCs are numbered from topologically-aggregatable
blocks that are assigned to a site at each point where it attaches
to the global Internet; where the topology is defined by the
connectivity of provider networks, RLOCs can be thought of as
Provider Aggregatable (PA) addresses. Routing for RLOCs is not
carried on the ALT.
EID-to-RLOC Mapping: A binding between an EID-prefix and the set of
RLOCs that can be used to reach it; sometimes referred to simply
as a "mapping".
EID-prefix Reachability: An EID-prefix is said to be "reachable" if
at least one of its locators is reachable. That is, an EID-prefix
is reachable if the ETR that is authoritative for a given EID-to-
RLOC mapping is reachable.
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Default Mapping: A Default Mapping is a mapping entry for EID-
prefix 0.0.0.0/0 (::/0 for ipv6). It maps to a locator-set used
for all EIDs in the Internet. If there is a more specific EID-
prefix in the mapping cache it overrides the Default Mapping
entry. The Default Mapping can be learned by configuration or
from a Map-Reply message.
ALT Default Route: An EID-prefix value of 0.0.0.0/0 (or ::/0 for
ipv6) which may be learned from the ALT or statically configured
on an edge ALT Router. The ALT-Default Route defines a forwarding
path for a packet to be sent into the ALT on a router which does
not have a full ALT forwarding database.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20113. The LISP+ALT model
The LISP+ALT model uses the same basic query/response protocol that
is documented in [LISP]. In particular, LISP+ALT provides two types
of packet that an ITR can originate to obtain EID-to-RLOC mappings:
Map-Request: A Map-Request message is sent into the ALT to request
an EID-to-RLOC mapping. The ETR which owns the mapping will
respond to the ITR with a Map-Reply message. Since the ALT only
forwards on EID destinations, the destination address of the Map-
Request sent on the ALT must be an EID.
Data Probe: Alternatively, an ITR may encapsulate and send the first
data packet destined for an EID with no known RLOCs into the ALT
as a Data Probe. This might be done to minimize packet loss and
to probe for the mapping. As above, the authoritative ETR for the
EID-prefix will respond to the ITR with a Map-Reply message when
it receives the data packet over the ALT. As a side-effect, the
encapsulated data packet is delivered to the end-system at the ETR
site. Note that the Data Probe's inner IP destination address,
which is an EID, is copied to the outer IP destination address so
that the resulting packet can be routed over the ALT. See
Section 3.3 for caveats on the usability of Data Probes.
The term "ALT Datagram" is short-hand for a Map-Request or Data Probe
to be sent into or forwarded on the ALT. Note that such packets use
an RLOC as the outer header source IP address and an EID as the outer
header destination IP address.
Detailed descriptions of the LISP packet types referenced by this
document may be found in [LISP].
3.1. Routeability of EIDs
A LISP EID has the same syntax as IP address and can be used,
unaltered, as the source or destination of an IP datagram. In
general, though, EIDs are not routable on the public Internet; LISP+
ALT provides a separate, virtual network, known as the LISP
Alternative Logical Topology (ALT) on which a datagram using an EID
as an IP destination address may be transmitted. This network is
built as an overlay on the public Internet using tunnels to
interconnect ALT Routers. BGP runs over these tunnels to propagate
path information needed to forward ALT Datagrams. Importantly, while
the ETRs are the source(s) of the unaggregated EID-prefixes, LISP+ALT
uses existing BGP mechanisms to aggregate this information.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20113.1.1. Mechanisms for an ETR to originate EID-prefixes
There are three ways that an ETR may originate its mappings into the
ALT:
1. By registration with a Map Server as documented in [LISP-MS].
This is the common case and is expected to be used by the
majority of ETRs.
2. Using a "static route" on the ALT. Where no Map-Server is
available, an edge ALT Router may be configured with a "static
EID-prefix route" pointing to an ETR.
3. Edge connection to the ALT. If a site requires fine- grained
control over how its EID-prefixes are advertised into the ALT, it
may configure its ETR(s) with tunnel and BGP connections to edge
ALT Routers.
3.1.2. Mechanisms for an ITR to forward to EID-prefixes
There are three ways that an ITR may send ALT Datagrams:
1. Through a Map Resolver as documented in [LISP-MS]. This is the
common case and is expected to be used by the majority of ITRs.
2. Using a "default route". Where a Map Resolver is not available,
an ITR may be configured with a static ALT Default Route pointing
to an edge ALT Router.
3. Edge connection to the ALT. If a site requires fine-grained
knowledge of what prefixes exist on the ALT, it may configure its
ITR(s) with tunnel and BGP connections to edge ALT Routers.
3.1.3. Map Server Model preferred
The ALT-connected ITR and ETR cases are expected to be rare, as the
Map Server/Map Resolver model is both simpler for an ITR/ETR operator
to use, and provides a more general service interface to not only the
ALT, but also to other mapping databases that may be developed in the
future.
3.2. Connectivity to non-LISP sites
As stated above, EIDs used as IP addresses by LISP sites are not
routable on the public Internet. This implies that, absent a
mechanism for communication between LISP and non-LISP sites,
connectivity between them is not possible. To resolve this problem,
an "interworking" technology has been defined; see [LISP-IW] for
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 2011
details.
3.3. Caveats on the use of Data Probes
It is worth noting that there has been a great deal of discussion and
controversy about whether Data Probes are a good idea. On the one
hand, using them offers a method of avoiding the "first packet drop"
problem when an ITR does not have a mapping for a particular EID-
prefix. On the other hand, forwarding data packets on the ALT would
require that it either be engineered to support relatively high
traffic rates, which is not generally feasible for a tunneled
network, or that it be carefully designed to aggressively rate-limit
traffic to avoid congestion or DoS attacks. There may also be issues
caused by different latency or other performance characteristics
between the ALT path taken by an initial Data Probe and the
"Internet" path taken by subsequent packets on the same flow once a
mapping is in place on an ITR. For these reasons, the use of Data
Probes is not recommended at this time; they should only be
originated an ITR when explicitly configured to do so and such
configuration should only be enabled when performing experiments
intended to test the viability of using Data Probes.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20114. LISP+ALT: Overview
LISP+ALT is a hybrid push/pull architecture. Aggregated EID-prefixes
are advertised among the ALT Routers and to those (rare) ITRs that
are directly connected via a tunnel and BGP to the ALT. Specific
EID-to-RLOC mappings are requested by an ITR (and returned by an ETR)
using LISP when it sends a request either via a Map Resolver or to an
edge ALT Router.
The basic idea embodied in LISP+ALT is to use BGP, running on a
tunneled overlay network (the ALT), to establish reachability between
ALT Routers. The ALT BGP Route Information Base (RIB) is comprised
of EID-prefixes and associated next hops. ALT Routers interconnect
using BGP and propagate EID-prefix updates among themselves. EID-
prefix information is learned from ETRs at the "edge" of the ALT
either through the use of the Map Server interface (the commmon
case), static configuration, or by BGP-speaking ETRs.
Map Resolvers learns paths through the ALT to Map Servers for EID-
prefixes. An ITR will normally use a Map Resolver to send its ALT
Datagrams on to the ALT but may, in unusual cases (see
Section 3.1.2), use a static ALT Default Route or connect to the ALT
using BGP. Likewise, an ETR will normally register its prefixes in
the mapping database using a Map Server can sometimes (see
Section 3.1.1) connect directly to the ALT using BGP. See [LISP-MS]
for details on Map Servers and Map Resolvers.
Note that while this document specifies the use of Generic Routing
Encapsulation (GRE) as a tunneling mechanism, there is no reason that
parts of the ALT cannot be built using other tunneling technologies,
particularly in cases where GRE does not meet security, management,
or other operational requirements. References to "GRE tunnel" in
later sections of this document should therefore not be taken as
prohibiting or precluding the use of other tunneling mechanisms.
Note also that two ALT Routers that are directly adjacent (with no
layer-3 router hops between them) need not use a tunnel between them;
in this case, BGP may be configured across the interfaces that
connect to their common subnet and that subnet is then considered to
be part of the ALT topology. Use of techniques such as "eBGP
multihop" to connect ALT Routers that do not share a tunnel or common
subnet is not recommended as the non-ALT Routers in between the ALT
Routers in such a configuration may not have information necessary to
forward ALT Datagrams destined to EID-prefixes exchanged across that
BGP session.
In summary, LISP+ALT uses BGP to build paths through ALT Routers so
that an ALT Datagram sent into the ALT can be forwarded to the ETR
that holds the EID-to-RLOC mapping for that EID-prefix. This
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 2011
reachability is carried as IPv4 or ipv6 NLRI without modification
(since an EID-prefix has the same syntax as IPv4 or ipv6 address
prefix). ALT Routers establish BGP sessions with one another,
forming the ALT. An ALT Router at the "edge" of the topology learns
EID-prefixes originated by authoritative ETRs. Learning may be
though the Map Server interface, by static configuration, or via BGP
with the ETRs. An ALT Router may also be configured to aggregate
EID-prefixes received from ETRs or from other LISP+ALT Routers that
are topologically "downstream" from it.
4.1. ITR traffic handling
When an ITR receives a packet originated by an end system within its
site (i.e. a host for which the ITR is the exit path out of the site)
and the destination EID for that packet is not known in the ITR's
mapping cache, the ITR creates either a Map-Request for the
destination EID or the original packet encapsulated as a Data Probe
(see Section 3.3 for caveats on the usability of Data Probes). The
result, known as an ALT Datagram, is then sent to an ALT Router (see
also [LISP-MS] for non-ALT-connected ITRs, noting that Data Probes
cannot be sent to a Map-Resolver). This "first hop" ALT Router uses
EID-prefix routing information learned from other ALT Routers via BGP
to guide the packet to the ETR which "owns" the prefix. Upon receipt
by the ETR, normal LISP processing occurs: the ETR responds to the
ITR with a LISP Map-Reply that lists the RLOCs (and, thus, the ETRs
to use) for the EID-prefix. For Data Probes, the ETR also
decapsulates the packet and transmits it toward its destination.
Upon receipt of the Map-Reply, the ITR installs the RLOC information
for a given prefix into a local mapping database. With these mapping
entries stored, additional packets destined to the given EID-prefix
are routed directly to an RLOC without use of the ALT, until either
the entry's TTL has expired, or the ITR can otherwise find no
reachable ETR. Note that a current mapping may exist that contains
no reachable RLOCs; this is known as a Negative Cache Entry and it
indicates that packets destined to the EID-prefix are to be dropped.
Full details on Map-Request/Map-Reply processing may be found in
[LISP].
Traffic routed on to the ALT consists solely of ALT Datagrams, i.e.
Map-Requests and Data Probes (if supported). Given the relatively
low performance expected of a tunneled topology, ALT Routers (and Map
Resolvers) should aggressively rate-limit the ingress of ALT
Datagrams from ITRs and, if possible, should be configured to not
accept packets that are not ALT Datagrams.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20114.2. EID Assignment - Hierarchy and Topology
The ALT database is organized in a herarchical manner with EID-
prefixs aggregated on power-of-2 block boundaries. Where a LISP site
has multiple EID-prefixes that are aligned on apower-of-2 block
boundary, they should be aggregated into a single EID-prefix for
advertisement. The ALT network is built in a roughly hierarchical,
partial mesh which is intended to allow aggregation where clearly-
defined hierarchical boundaries exist. Building such a structure
should minimize the number of EID-prefixes carried by LISP+ALT nodes
near the top of the hierarchy.
Routes on the ALT do not need to respond to changes in policy,
subscription, or underlying physical connectivity, so the topology
can remain relatively static and aggregation can be sustained.
Because routing on the ALT uses BGP, the same rules apply for
generating aggregates; in particular, a ALT Router should only be
configured to generate an aggregate if it is configured with BGP
sessions to all of the originators of components (more-specific
prefixes) of that aggregate. Not all of the components of need to be
present for the aggregate to be originated (some may be holes in the
covering prefix and some may be down) but the aggregating router must
be configured to learn the state of all of the components.
Under what circumstances the ALT Router actually generates the
aggregate is a matter of local policy: in some cases, it will be
statically configured to do so at all times with a "static discard"
route. In other cases, it may be configured to only generate the
aggregate prefix if at least one of the components of the aggregate
is learned via BGP.
An ALT Router must not generate an aggregate that includes a non-
LISP-speaking hole unless it can be configured to return a Negative
Map-Reply with action="Natively-Forward" (see [LISP]) if it receives
an ALT Datagram that matches that hole. If it receives an ALT
Datagram that matches a LISP-speaking hole that is currently not
reachable, it should return a Negative Map-Reply with action="drop".
Negative Map-Replies should be returned with a short TTL, as
specified in [LISP-MS]. Note that an off-the-shelf, non-LISP-
speaking router configured as an aggregating ALT Router cannot send
Negative Map-Replies, so such a router must never originate an
aggregate that includes a non-LISP-speaking hole.
This implies that two ALT Routers that share an overlapping set of
prefixes must exchange those prefixes if either is to generate and
export a covering aggregate for those prefixes. It also implies that
an ETR which connects to the ALT using BGP must maintain BGP sessions
with all of the ALT Routers that are configured to originate an
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aggregate which covers that prefix and that each of those ALT Routers
must be explicitly configured to know the set of EID-prefixes that
make up any aggregate that it originates. See also [LISP-MS] for an
example of other ways that prefix origin consistency and aggregation
can be maintained.
As an example, consider ETRs that are originating EID-prefixes for
10.1.0.0/24, 10.1.64.0/24, 10.1.128.0/24, and 10.1.192.0/24. An ALT
Router should only be configured to generate an aggregate for
10.1.0.0/16 if it has BGP sessions configured with all of these ETRs,
in other words, only if it has sufficient knowledge about the state
of those prefixes to summarize them. If the Router originating
10.1.0.0/16 receives an ALT Datagram destined for 10.1.77.88, a non-
LISP destination covered by the aggregate, it returns a Negative Map-
Reply with action "Natively-Forward". If it receives an ALT Datagram
destined for 10.1.128.199 but the configured LISP prefix
10.1.128.0/24 is unreachable, it returns a Negative Map-Reply with
action "drop".
Note: much is currently uncertain about the best way to build the ALT
network; as testing and prototype deployment proceeds, a guide to how
to best build the ALT network will be developed.
4.3. Use of GRE and BGP between LISP+ALT Routers
The ALT network is built using GRE tunnels between ALT Routers. BGP
sessions are configured over those tunnels, with each ALT Router
acting as a separate AS "hop" in a Path Vector for BGP. For the
purposes of LISP+ALT, the AS-path is used solely as a shortest-path
determination and loop-avoidance mechanism. Because all next-hops
are on tunnel interfaces, no IGP is required to resolve those next-
hops to exit interfaces.
LISP+ALT's use of GRE and BGP facilities deployment and operation of
LISP because no new protocols need to be defined, implemented, or
used on the overlay topology; existing BGP/GRE tools and operational
expertise are also re-used. Tunnel address assignment is also easy:
since the addresses on an ALT tunnel are only used by the pair of
routers connected to the tunnel, the only requirement of the IP
addresses used to establish that tunnel is that the attached routers
be reachable by each other; any addressing plan, including private
addressing, can therefore be used for ALT tunnels.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20115. EID-prefix Propagation and Map-Request Forwarding
As described in Section 8.2, an ITR sends an ALT Datagram to a given
EID-to-RLOC mapping. The ALT provides the infrastructure that allows
these requests to reach the authoritative ETR.
Note that under normal circumstances Map-Replies are not sent over
the ALT; an ETR sends a Map-Reply to one of the ITR RLOCs learned
from the original Map-Request. See sections 6.1.2 and 6.2 of [LISP]
for more information on the use of the Map-Request ITR RLOC field.
Keep in mind that the ITR RLOC field supports mulitple RLOCs in
multiple address families, so a Map-Reply sent in response to a Map-
Request is not necessarily sent to back to the Map-Request RLOC
source.
There may be scenarios, perhaps to encourage caching of EID-to-RLOC
mappings by ALT Routers, where Map-Replies could be sent over the ALT
or where a "first-hop" ALT Router might modify the originating RLOC
on a Map-Request received from an ITR to force the Map-Reply to be
returned to the "first-hop" ALT Router. These cases will not be
supported by initial LISP+ALT implementations but may be subject to
future experimentation.
ALT Routers propagate path information via BGP ([RFC4271]) that is
used by ITRs to send ALT Datagrams toward the appropriate ETR for
each EID-prefix. BGP is run on the inter-ALT Router links, and
possibly between an edge ("last hop") ALT Router and an ETR or
between an edge ("first hop") ALT Router and an ITR. The ALT BGP RIB
consists of aggregated EID-prefixes and their next hops toward the
authoritative ETR for that EID-prefix.
5.1. Changes to ITR behavior with LISP+ALT
As previously described, an ITR will usually use the Map Resolver
interface and will send its Map Requests to a Map Resolver. When an
ITR instead connects via tunnels and BGP to the ALT, it sends ALT
Datagrams to one of its "upstream" ALT Routers; these are sent only
to obtain new EID-to-RLOC mappings - RLOC probe and cache TTL refresh
Map-Requests are not sent on the ALT. As in basic LISP, it should
use one of its RLOCs as the source address of these queries; it
should not use a tunnel interface as the source address as doing so
will cause replies to be forwarded over the tunneled topology and may
be problematic if the tunnel interface address is not routed
throughout the ALT. If the ITR is running BGP with the LISP+ALT
router(s), it selects the appropriate ALT Router based on the BGP
information received. If it is not running BGP, it uses a
statically-configued ALT Default Route to select an ALT Router.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20115.2. Changes to ETR behavior with LISP+ALT
As previously described, an ETR will usually use the Map Server
interface (see [LISP-MS]) and will register its EID-prefixes with its
configured Map Servers. When an ETR instead connects using BGP to
one or more ALT Routers, it announces its EID-prefix(es) to those ALT
Routers.
As documented in [LISP], when an ETR generates a Map-Reply message to
return to a querying ITR, it sets the outer header IP destination
address to one of the requesting ITR's RLOCs so that the Map-Reply
will be sent on the underlying Internet topology, not on the ALT;
this avoids any latency penalty (or "stretch") that might be incurred
by sending the Map-Reply via the ALT, reduces load on the ALT, and
ensures that the Map-Reply can be routed even if the original ITR
does not have an ALT-routed EID. For details on how an ETR selects
which ITR RLOC to use, see section 6.1.5 of [LISP].
5.3. ALT Datagram forwarding falure
Intermediate ALT Routers, forward ALT Datagrams using normal, hop-by-
hop routing on the ALT overlay network. Should an ALT router not be
able to forward an ALT Datagram, whether due to an unreachable next-
hop, TTL exceeded, or other problem, it has several choices:
o If the ALT Router understands the LISP protocol, as is the case
for a Map Resolver or Map Server, it may respond to a forwarding
failure by returning a negative Map-Reply, as described in
Section 4.2 and [LISP-MS].
o If the ALT Router does not understand LISP, it may attempt to
return an ICMP message to the source IP address of the packet that
cannot be forwarded. Since the source address is an RLOC, an ALT
Router would send this ICMP message using "native" Internet
connectivity, not via the ALT overlay.
o A non-LISP-capable ALT Router may also choose to silently drop the
non-forwardable ALT Datagram.
[LISP] and [LISP-MS] define how the source of an ALT Datagram should
handle each of these cases. The last case, where an ALT Datagram is
silently discarded, will generally result in several retransmissions
by the source, followed by treating the destination as unreachable
via LISP when no Map-Reply is received. If a problem on the ALT is
severe enough to prevent ALT Datagrams from being delivered to a
specific EID, this is probably the only sensible way to handle this
case.
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Note that the use of GRE tunnels should prevent MTU problems from
ever occurring on the ALT; an ALT Datagram that exceeds an
intermediate MTU will be fragmented at that point and will be
reassembled by the target of the GRE tunnel.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20116. BGP configuration and protocol considerations6.1. Autonomous System Numbers (ASNs) in LISP+ALT
The primary use of BGP today is to define the global Internet routing
topology in terms of its participants, known as Autonomous Systems.
LISP+ALT specifies the use of BGP to create a global overlay network
(the ALT) for finding EID-to-RLOC mappings. While related to the
global routing database, the ALT serves a very different purpose and
is organized into a very different hierarchy. Because LISP+ALT does
use BGP, however, it uses ASNs in the paths that are propagated among
ALT Routers. To avoid confusion, LISP+ALT should use newly-assigned
AS numbers that are unrelated to the ASNs used by the global routing
system. Exactly how this new space will be assigned and managed will
be determined during the deployment of LISP+ALT.
Note that the ALT Routers that make up the "core" of the ALT will not
be associated with any existing core-Internet ASN because the ALT
topology is completely separate from, and independent of, the global
Internet routing system.
6.2. Sub-Address Family Identifier (SAFI) for LISP+ALT
As defined by this document, LISP+ALT may be implemented using BGP
without modification. Given the fundamental operational difference
between propagating global Internet routing information (the current
dominant use of BGP) and creating an overlay network for finding EID-
to-RLOC mappings (the use of BGP proposed by this document), it may
be desirable to assign a new SAFI [RFC4760] to prevent operational
confusion and difficulties, including the inadvertent leaking of
information from one domain to the other. Use of a separate SAFI
would make it easier to debug many operational problems but would
come at a significant cost: unmodified, off-the-shelf routers which
do not understand the new SAFI could not be used to build any part of
the ALT network. At present, this document does not request the
assignment of a new SAFI; additional experimentation may suggest the
need for one in the future.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20117. EID-prefix Aggregation
The ALT BGP peering topology should be arranged in a tree-like
fashion (with some meshiness), with redundancy to deal with node and
link failures. A basic assumption is that as long as the routers are
up and running, the underlying Internet will provide alternative
routes to maintain BGP connectivity among ALT Routers.
Note that, as mentioned in Section 4.2, the use of BGP by LISP+ALT
requires that information only be aggregated where all active more-
specific prefixes of a generated aggregate prefix are known. This is
no different than the way that BGP route aggregation works in the
existing global routing system: a service provider only generates an
aggregate route if it is configured to learn to all prefixes that
make up that aggregate.
7.1. Stability of the ALT
It is worth noting that LISP+ALT does not directly propagate EID-to-
RLOC mappings. What it does is provide a mechanism for an ITR to
commonicate with the ETR that holds the mapping for a particular EID-
prefix. This distinction is important when considering the stability
of BGP on the ALT network as compared to the global routing system.
It also has implications for how site-specific EID-prefix information
may be used by LISP but not propagated by LISP+ALT (see Section 7.2
below).
RLOC prefixes are not propagated through the ALT so their
reachability is not determined through use of LISP+ALT. Instead,
reachability of RLOCs is learned through the LISP ITR-ETR exchange.
This means that link failures or other service disruptions that may
cause the reachability of an RLOC to change are not known to the ALT.
Changes to the presence of an EID-prefix on the ALT occur much less
frequently: only at subscription time or in the event of a failure of
the ALT infrastructure itself. This means that "flapping" (frequent
BGP updates and withdrawals due to prefix state changes) is not
likely and mapping information cannot become "stale" due to slow
propagation through the ALT BGP mesh.
7.2. Traffic engineering using LISP
Since an ITR learns an EID-to-RLOC mapping directly from the ETR that
owns it, it is possible to perform site-to-site traffic engineering
by setting the preference and/or weight fields, and by including
more-specific EID-to-RLOC information in Map-Reply messages.
This is a powerful mechanism that can conceivably replace the
traditional practice of routing prefix deaggregation for traffic
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 2011
engineering purposes. Rather than propagating more-specific
information into the global routing system for local- or regional-
optimization of traffic flows, such more-specific information can be
exchanged, through LISP (not LISP+ALT), on an as-needed basis between
only those ITRs/ETRs (and, thus, site pairs) that need it. Such an
exchange of "more-specifics" between sites facilitates traffic
engineering, by allowing richer and more fine-grained policies to be
applied without advertising additional prefixes into either the ALT
or the global routing system.
Note that these new traffic engineering capabilities are an attribute
of LISP and are not specific to LISP+ALT; discussion is included here
because the BGP-based global routing system has traditionally used
propagation of more-specific routes as a crude form of traffic
engineering.
7.3. Edge aggregation and dampening
Normal BGP best common practices apply to the ALT network. In
particular, first-hop ALT Routers will aggregate EID prefixes and
dampen changes to them in the face of excessive updates. Since EID-
prefix assignments are not expected to change as frequently as global
routing BGP prefix reachability, such dampening should be very rare,
and might be worthy of logging as an exceptional event. It is again
worth noting that the ALT carries only EID-prefixes, used to a
construct BGP path to each ETR (or Map-Server) that originates each
prefix; the ALT does not carry reachability about RLOCs. In
addition, EID-prefix information may be aggregated as the topology
and address assignment hierarchy allow. Since the topology is all
tunneled and can be modified as needed, reasonably good aggregation
should be possible. In addition, since most ETRs are expected to
connect to the ALT using the Map Server interface, Map Servers will
implement a natural "edge" for the ALT where dampening and
aggregation can be applied. For these reasons, the set of prefix
information on the ALT can be expected to be both better aggregated
and considerably less volatile than the actual EID-to-RLOC mappings.
7.4. EID assignment flexibility vs. ALT scaling
There are major open questions regarding how the ALT will be deployed
and what organization(s) will operate it. In a simple, non-
distributed world, centralized administration of EID prefix
assignment and ALT network design would facilitate a well- aggregated
ALT routing system. Business and other realities will likely result
in a more complex, distributed system involving multiple levels of
prefix delegation, multiple operators of parts of the ALT
infrastructure, and a combination of competition and cooperation
among the participants. In addition, re-use of existing IP address
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 2011
assignments, both provider-independent ("PI") and provider-assigned
("PA"), to avoid renumbering when sites transition to LISP will
further complicate the processes of building and operating the ALT.
A number of conflicting considerations need to be kept in mind when
designing and building the ALT. Among them are:
1. Target ALT routing state size and level of aggregation. As
described in Section 7.1, the ALT should not suffer from some of
the performance constraints or stability issues as the Internet
global routing system, so some reasonable level of deaggregation
and increased number of EID prefixes beyond what might be
considered ideal should be acceptable. That said, measures, such
as tunnel rehoming to preserve aggregation when sites move from
one mapping provider to another and implementing aggregation at
multiple levels in the hierarchy to collapse de-aggregation at
lower levels, should be taken to reduce unnecessary explosion of
ALT routing state.
2. Number of operators of parts of the ALT and how they will be
organized (hierarchical delegation vs. shared administration).
This will determine not only how EID prefixes are assigned but
also how tunnels are configured and how EID prefixes can be
aggregated between different parts of the ALT.
3. Number of connections between different parts of the ALT. Trade-
offs will need to be made among resilience, performance, and
placement of aggregation boundaries.
4. EID prefix portability between competing operators of the ALT
infrastructure. A significant benefit for an end-site to adopt
LISP is the availability of EID space that is not tied to a
specific connectivity provider; it is important to ensure that an
end site doesn't trade lock-in to a connectivity provider for
lock-in to a provider of its EID assignment, ALT connectivity, or
Map Server facilities.
This is, by no means, an exhaustive list.
While resolving these issues is beyond the scope of this document,
the authors recommend that existing distributed resource structures,
such as the IANA/Regional Internet Registries and the ICANN/Domain
Registrar, be carefully considered when designing and deploying the
ALT infrastructure.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 20118. Connecting sites to the ALT network8.1. ETRs originating information into the ALT
EID-prefix information is originated into the ALT by three different
mechanisms:
Map Server: In most cases, a site will configure its ETR(s) to
register with one or more Map Servers (see [LISP-MS]), and does
not participate directly in the ALT.
BGP: For a site requiring complex control over their EID-prefix
origination into the ALT, an ETR may connect to the LISP+ALT
overlay network by running BGP to one or more ALT Router(s) over
tunnel(s). The ETR advertises reachability for its EID-prefixes
over these BGP connection(s). The edge ALT Router(s) that
receive(s) these prefixes then propagate(s) them into the ALT.
Here the ETR is simply an BGP peer of ALT Router(s) at the edge of
the ALT. Where possible, an ALT Router that receives EID-prefixes
from an ETR via BGP should aggregate that information.
Configuration: One or more ALT Router(s) may be configured to
originate an EID-prefix on behalf of the non-BGP-speaking ETR that
is authoritative for a prefix. As in the case above, the ETR is
connected to ALT Router(s) using GRE tunnel(s) but rather than BGP
being used, the ALT Router(s) are configured with what are in
effect "static routes" for the EID-prefixes "owned" by the ETR.
The GRE tunnel is used to route Map-Requests to the ETR.
Note: in all cases, an ETR may register to multiple Map Servers or
connect to multiple ALT Routers for the following reasons:
* redundancy, so that a particular ETR is still reachable even if
one path or tunnel is unavailable.
* to connect to different parts of the ALT hierarchy if the ETR
"owns" multiple EID-to-RLOC mappings for EID-prefixes that
cannot be aggregated by the same ALT Router (i.e. are not
topologically "close" to each other in the ALT).
8.2. ITRs Using the ALT
In the common configuration, an ITR does not need to know anything
about the ALT, since it sends Map-Requests to one of its configured
Map-Resolvers (see [LISP-MS]). There are two exceptional cases:
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 2011
Static default: If a Map Resolver is not available but an ITR is
adjacent to an ALT Router (either over a common subnet or through
the use of a tunnel), it can use an ALT Default Route route to
cause all ALT Datagrams to be sent that ALT Router. This case is
expected to be rare.
Connection to ALT: A site with complex Internet connectivity needs
may need more fine-grained distinction between traffic to LISP-
capable and non-LISP-capable sites. Such a site may configure
each of its ITRs to connect directly to the ALT, using a tunnel
and BGP connection. In this case, the ITR will receive EID-prefix
routes from its BGP connection to the ALT Router and will LISP-
encapsulate and send ALT Datagrams through the tunnel to the ALT
Router. Traffic to other destinations may be forwarded (without
LISP encapsulation) to non-LISP next-hop routers that the ITR
knows.
In general, an ITR that connects to the ALT does so only to to ALT
Routers at the "edge" of the ALT (typically two for redundancy).
There may, though, be situations where an ITR would connect to
other ALT Routers to receive additional, shorter path information
about a portion of the ALT of interest to it. This can be
accomplished by establishing GRE tunnels between the ITR and the
set of ALT Routers with the additional information. This is a
purely local policy issue between the ITR and the ALT Routers in
question.
As described in [LISP-MS], Map-Resolvers do not accept or forward
Data Probes; in the rare scenario that an ITR does support and
originate Data Probes, it must do so using one of the exceptional
configurations described above. Note that the use of Data Probes is
discouraged at this time (see Section 3.3).
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 201110. Security Considerations
LISP+ALT shares many of the security characteristics of BGP. Its
security mechanisms are comprised of existing technologies in wide
operational use today, so securing the ALT should be mostly a matter
of applying the same technology that is used to secure the BGP-based
global routing system (see Section 10.3 below).
10.1. Apparent LISP+ALT Vulnerabilities
This section briefly lists the known potential vulnerabilities of
LISP+ALT.
Mapping Integrity: Potential for an attacker to insert bogus
mappings to black-hole (create Denial-of-Service, or DoS attack)
or intercept LISP data-plane packets.
ALT Router Availability: Can an attacker DoS the ALT Routers
connected to a given ETR? If a site's ETR cannot advertise its
EID-to-RLOC mappings, the site is essentially unavailable.
ITR Mapping/Resources: Can an attacker force an ITR or ALT Router to
drop legitimate mapping requests by flooding it with random
destinations for which it will generate large numbers of Map-
Requests and fill its mapping cache? Further study is required to
see the impact of admission control on the overlay network.
EID Map-Request Exploits for Reconnaissance: Can an attacker learn
about a LISP site's TE policy by sending legitimate mapping
requests and then observing the RLOC mapping replies? Is this
information useful in attacking or subverting peer relationships?
Note that any public LISP mapping database will have similar data-
plane reconnaissance issue.
Scaling of ALT Router Resources: Paths through the ALT may be of
lesser bandwidth than more "direct" paths; this may make them more
prone to high-volume denial-of-service attacks. For this reason,
all components of the ALT (ETRs and ALT Routers) should be
prepared to rate-limit traffic (ALT Datagrams) that could be
received across the ALT.
UDP Map-Reply from ETR: Since Map-Replies are sent directly from the
ETR to the ITR's RLOC, the ITR's RLOC may be vulnerable to various
types of DoS attacks (this is a general property of LISP, not an
LISP+ALT vulnerability).
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More-specific prefix leakage: Because EID-prefixes on the ALT are
expected to be fairly well-aggregated and EID-prefixes propagated
out to the global Internet (see [LISP-IW]) much more so,
accidental leaking or malicious advertisement of an EID-prefix
into the global routing system could cause traffic redirection
away from a LISP site. This is not really a new problem, though,
and its solution can only be achieved by much more strict prefix
filtering and authentication on the global routing system.
Section Section 10.3 describes an existingapproach to solving this
problem.
10.2. Survey of LISP+ALT Security Mechanisms
Explicit peering: The devices themselves can both prioritize
incoming packets, as well as potentially do key checks in hardware
to protect the control plane.
Use of TCP to connect elements: This makes it difficult for third
parties to inject packets.
Use of HMAC to protect BGP/TCP connections: HMAC [RFC5925] is used
to verify the integrity and authenticity of TCP connections used
to exchange BGP messages, making it nearly impossible for third
party devices to either insert or modify messages.
Message sequence numbers and nonce values in messages: This allows
an ITR to verify that the Map-Reply from an ETR is in response to
a Map-Request originated by that ITR (this is a general property
of LISP; LISP+ALT does not change this behavior).
10.3. Use of new IETF standard BGP Security mechanisms
LISP+ALT's use of BGP allows it to take advantage of BGP security
features designed for existing Internet BGP use. This means that
LISP+ALT can and should use technology developed for adding security
to BGP (in the IETF SIDR working group or elsewhere) to provide
authentication of EID-prefix origination and EID-to-RLOC mappings.
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Internet-Draft LISP Alternative Topology (LISP+ALT) December 201111. Acknowledgments
The authors would like to specially thank J. Noel Chiappa who was a
key contributer to the design of the LISP-CONS mapping database (many
ideas from which made their way into LISP+ALT) and who has continued
to provide invaluable insight as the LISP effort has evolved. Others
who have provided valuable contributions include John Zwiebel, Hannu
Flinck, Amit Jain, John Scudder, Scott Brim, and Jari Arkko.
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